The present invention relates, in general, to chemical mechanical polishing and, more particularly, to compositions which are used as polishing slurries in the process of polishing silicon wafers for the semiconductor industry.
Silicon wafers for the semiconductor industry must possess a high degree of surface perfection before they can be useful in the device fabrication process. These surfaces are universally produced by polishing the wafer with a polishing composition (hereinafter sometimes referred to as a xe2x80x9cslurryxe2x80x9d from its nature). Polishing slurries generally consist of a composition which contains a concentration of submicron particles. The part, or substrate, is bathed or rinsed in the slurry in conjunction with an elastomeric pad which is pressed against the substrate and rotated such that the slurry particles are pressed against the substrate under load. The lateral motion of the pad causes the slurry particles to move across the substrate surface, resulting in wear, or volumetric removal of the substrate surface. Ideally, this process results in the selective erosion of projecting surface features so that when the process is completed a perfect plane surface is produced down to the finest level of detail.
The silicon polishing process as practiced in industry consists of two or more steps. In the first, or coarse polish step, gross defects remaining from wafer sawing and shaping operations are removed. The wafer surface appears smooth and specular but still contains numerous minute defects. These defects are removed by subsequent final polish steps which remove more material from the surface and act to polish away the surface defects thereby minimizing the surface roughness and producing a low haze surface.
The measurement of surface roughness and surface defect/particle concentrations on polished wafers is generally performed by a scanning light scattering detector. Various models; e.g. Censor ANS100, Tencor 6200 and Estek WIS9000; are widely used in the silicon wafer industry. All detectors utilize the same principle of operation, namely that they measure the degree of non-specular reflected light from the wafer surface. A high intensity laser beam is scanned across the surface of the wafer. Non-specular reflected light is collected in an off-axis detector and the signal intensity of this scattered light is analyzed. Surface roughness results in generalized light scattering of low intensity, generally termed haze. Particles or other discrete surface imperfections produce a more intense scattering which has a point source origin. The intensity of these point sources of scattering is ranked in comparison to that of latex standard calibration spheres of varying sizes. These point sources are generally refereed to as Light Point Defects, or LPDs and their distribution is ranked according to measurement technology and terminology is given by P. O. Hahn et al. in an article entitled xe2x80x9cThe Sixe2x80x94SiO2 Interface Roughness: Causes and Effectsxe2x80x9d in C. R. Helms and B. E. Dead, Eds., xe2x80x9cThe Physics and Chemistry of SiO2 and the Sixe2x80x94SiO2 Interface, pp. 401-411, Plenum Press, New York (1988) incorporated by reference herein. So far as the present inventors are aware, in the conventional two step polishing, it was possible to obtain a substrate surface having a good surface roughness in the polishing in the second step, but the stock removal rate was very low.
For the final polishing step, particularly in finishing polishing in two steps, it has been common to carry out polishing by means of a polishing composition prepared in such a manner that aluminum oxide or other abrasive is thoroughly pulverized and adjusted for proper particle size, water is added thereto, and aluminum nitrate or various organic acids and other polishing accelerators are incorporated thereto, or a polishing composition comprising colloidal silica and water is used. However, the polishing by means of the former polishing composition had a problem that the balance between the mechanical component and the chemical component was poor, and microprotusions or fine pits tended to form. The polishing by means of the latter polishing composition had a problem such that the stock removal rate was so low that it took a long time for polishing, the productivity was low and it has been difficult to prevent formation of microprotrusions, fine pits and other surface defects.
Accordingly, it has been desired to develop a polishing composition or a surface treating composition that has a high polishing removal rate and that is capable of forming a very smooth polished surface with a low haze.
There are numerous background patents on slurries to be used for polishing silicon wafers.
Minoru and Yutaka (Japanese Unexamined Patent Publication No. 11-116942) describes a polishing slurry for silicon wafers consisting of a silicon dioxide, water, water-soluble polymer compound, basic compound, alcoholic hydroxy group compound, and hydroxy group nitrogen containing compound. No criticality of high molecular weight and high polydispersity of the water-soluble polymer compound was taught. Furthermore, it teaches the use of hydroxyethylcellulose in combination with poly vinyl alcohol but does not teach the use of hydroxyethylcellulose alone.
Miyishata and Minami (European Patent Application EP 0 933 166 A1) describes a polishing agent consisting of abrasive containing silica particles as the main component, water as a solvent, and a water-soluble cellulose. No criticality of high molecular weight and high polydispersity is taught.
The present invention provides a polishing composition which has a high polishing removal rate and which is cable of forming a very smooth polished surface with low haze.
In one aspect the invention is a polishing composition comprising peanut-like shaped colloidal silica particles, water-soluble cellulose, ammonia and water.
In another aspect the invention is a polishing composition comprising an abrasive, water-soluble cellulose having a molecular weight of at least about 1,000,000, an alkaline compound and water.
In yet another aspect the invention is a polishing composition comprising an abrasive, water-soluble cellulose having a polydispersity of at least about 5 MW/Mn, an alkaline compound and water.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, the invention being defined only by the claims following this detailed description.